The present invention relates to novel methods and materials for minimizing metal-oxide deposits on steam generator tubes in the secondary side of pressurized nuclear steam generators by utilizing specific high-purity polymer dispersants.
At present, no method or process exists for eliminating and preventing the deposit of metal-oxides/sludge in the secondary side of nuclear steam generators during operation of the generator. The only method and process existing for controlling the amount of impurities that enter into the secondary side of the steam generator is the utilization of pure water. The consequences resulting from the buildup of metal oxides within the secondary side of a steam generator are reduced steam output thereby resulting in lost electrical output from the generating plant, increased water level fluctuations within the steam generator thereby resulting in lower steam and electrical output, and the initiation of corrosion deposits within the heat exchanger through the concentration of the dissolved chemical species from the secondary water. The corrosion within the secondary side of a pressurized nuclear steam generator ultimately may result in tube plugging and sleeving and the eventual loss of electrical output because of lost heat transfer or flow imbalances unless the steam generators themselves are replaced at a cost of approximately $ 200,000,000 per plant.
Accordingly, all known processes for eliminating deposits of metal oxides in the secondary side of recirculating steam generators have been directed to the removal of these deposits after they build-up in the heat exchanger. The major technique utilized for the removal of suspended and dissolved impurities from the secondary side of the recirculating steam generator involves removing a portion of the water from the steam generator during operation on a continuous or periodic basis through a blowdown system. Typically, the blowdown system only removes up to 10 percent of the total metal oxides or impurities which enter the recirculating nuclear system generator during operation, with the remaining metal oxides or impurities continuing to build-up and to be deposited within the secondary side of the recirculating nuclear steam generator. This deposition may result in pressure loss, level fluctuations, and corrosion of the secondary side of the nuclear steam generator.
Several mechanical and chemical methods have been suggested for removing metal oxides or impurities from within the secondary side of nuclear steam generators when the system is near or at shutdown conditions. One of these methods utilizes sludge lancing at shutdown which employs high pressure water to flush loosely adhered oxide deposits and sludge from the lower tube sheet of the nuclear steam generator. This process typically does not address deposition of corrosion in the upper tube support plates and does not clean any clogged crevices on the secondary side of the nuclear steam generator. The percentage of metal oxides or corrosion removed by this process is about two percent of the total oxides entering the nuclear steam generators over a typical 18-month fuel cycle. The cost of completing a sludge lancing is approximately $350,000 for each 18-month fuel cycle in a typical four-loop plant.
Another method suggested for removing metal oxides/sludge at shutdown from the secondary side of a nuclear steam generator is the bundle-flush process. This process entails directing flush water from the upper part of the recirculating nuclear steam generator to remove the loose sludge from the upper tube support plates. The cost of the bundle flush process is approximately $500,000 per application; however, the process only removes the soft, loosely adhered sludge, and does not remove sludge which is strongly adhered to the heat transfer surfaces. Additionally, the small crevices within the heat transfer structure are not cleaned at all by this process. Accordingly, this process is of limited value and does not overcome the problem of removing strongly adhered deposits or impediments within the heat-transfer structure.
Crevice flush techniques have been suggested in an attempt to open or clean closed or packed crevices by heating the secondary side of the nuclear steam generator above a boiling point with an inert atmosphere overpressure and then releasing this overpressure. The crevice flush process results in a boiling action which purportedly flushes the impurities from the crevices in the nuclear steamed generator. However, this method has only demonstrated limited effectiveness and is very time consuming, thereby prolonging downtime, an added cost in the electrical industry.
Chemical-soak techniques have been suggested for use during shutdown to promote removal of loose sludge and loosely adhered deposits within the nuclear steam generator. The chemical soaks employ amines such as dimethylamine and morpholine. These soaks have exhibited limited effectiveness in removing loosely adhered deposits, and the amount or percentage of metal oxides removed is less than acceptable. The advantage of this process is that the cost is low; but the disadvantages of this method are that the process is time consuming, and the effectiveness and the amount of metal oxides removed is less than satisfactory.
Pressure-pulse cleaning or water slapping are mechanical methods which are utilized during an outage or shutdown for removing loosely adhered sludge from the upper tubes or the tube support plates of the nuclear steam generator. The sludge or deposits are removed by raising the water on the secondary side to a desired level and then injecting a high pressure gas such as nitrogen into the water. The bursting of the bubbles as the gas approaches the surface of the water partially removes limited amounts of the loosely adhered sludge or oxide deposits. This technique may increase the amount of metal oxides removed from 5-15 percent of the total amount of metal oxides deposited within the nuclear steam generator; however, this method does not remove hard deposits and does not open crevices packed with metal oxides or other corrosion. The cost of a pressure pulse cleaning is typically $200,000 to $600,000 per unit. It is recommended that such a cleaning be employed every one-to-four refueling cycles.
Finally, the methods of chemical cleaning at low or high temperatures and the use of chemically enhanced pressure pulse cleaning are processes utilizing specific organic materials that dissolve the metal-oxide deposits within the nuclear steam generator. The cleaning solution dissolves the metal-oxide deposits, and the spent cleaning solution must be processed and properly disposed. The chemical-cleaning processes may be selected to remove specific metal oxides contained within the nuclear steam generator. Variations of the chemical cleaning process include the heating of the cleaning solution above the liquid-boiling temperature under an inert atmosphere and then releasing the pressure to force boiling in the cracks and the crevices and the use of pulse-cleaning techniques to promote circulation and movement of the cleaning solution. The chemical cleaning processes remove virtually one-hundred percent of the metal-oxide deposits within the secondary side of the recirculating steam generator, but at a cost of between $5,000,000-$10,000,000 per cleaning. Many of the nuclear generating plants in operation may require chemical cleaning at least once during their lifetime.
Thus, each of the mechanical prior art methods for removing metal-oxides from the secondary side of the nuclear steam generator is directed to removing the loosely deposited oxides within the heat-exchange structure that results from the continued operation of the nuclear power plant. Although chemical cleaning removes substantially all metal oxides, such a process is extremely expensive and time consuming. Accordingly, none of the known chemical or mechanical methods is directed to preventing the deposition or formation of sludge within the secondary side of a nuclear steam generator during operation of the generator. These processes attempt to remove the oxide and corrosive deposits after they have been deposited in the secondary side of the nuclear steam generator, processes which are extremely costly and which result in significant downtime of the nuclear power plant.
Natural polymer dispersants have been used to minimize deposition of sludge deposits in fossil steam generators since the early 1900""s, and synthetic polymers have been recently utilized for metal-oxide dispersing and sludge conditioning in fossil steam generators. However, such synthetic polymers have not been qualified for use in minimizing metal-oxide deposition on the secondary side of recirculating nuclear steam generators. Most synthetic polymers developed and used today in water-treatment applications are manufactured using inorganics, such as sodium persulfate, as the initiators of polymerization, and other inorganics as chain transfer agents. However, the sodium and persulfate inorganics contribute unwanted contaminants in significant excess to those required for application in nuclear steam generator units. Polymers typically used in boilers contain inorganic solids at concentrations up to 500 times the allowable levels for application to nuclear steam generator units. Inorganic impurities can include sodium, potassium, chlorine, sulfur, fluorine, and phosphorusxe2x80x94elements which are particularly objectionable and damaging when used in nuclear steam generator operations.
Synthetic polymers used in water treatment applications are typically neutralized with sodium or potassium, forming the inorganic salt. Although ammonia-neutralized versions have been used to a small extent, ammonia is a known copper-alloy corrodent. Polymer neutralization minimizes system upset potential. Polymers have been used unneutralized, but the feed-rate variations have been known to cause system upsets by lowering the pH, thereby resulting in corrosion to the operating system.
It is one object of the present invention to provide a method and process of substantially preventing the formation of sludge, corrosion, or metal-oxide deposits within the secondary side of nuclear steam generators during all phases of operation.
It is another object of the present invention to provide a method of substantially preventing the formation of sludge, corrosion, or metal-oxide deposits within the secondary side of nuclear steam generators during all phases of operation by utilizing the application of a high-purity polymer dispersant to the feedwater entering the secondary side of nuclear steam generators.
It is still another object of the present invention to apply a high-purity polymer dispersant to the feedwater entering the secondary side of nuclear steam generators, wherein the high purity polymer dispersant is selected from a group consisting of acrylic acid polymer, methacrylic acid polymer, acrylate polymer, methacrylate polymer, copolymers, terpolymers, and mixtures thereof.
It is still another object of the present invention to select a high-purity polymer dispersant from a group consisting of acrylate/acrylamide copolymer, acrylate/methacrylate copolymer, terpolymers, and mixtures thereof.
It is still another object of the present invention to utilize qualified high-purity polymer dispersants added to the feedwater entering the secondary side of nuclear steam generators to prevent the formation of sludge, corrosion, or metal-oxide deposits within the secondary side of nuclear steam generators.
It is still another object of the present invention to utilize a chemically pure polymer dispersant combination to remove metal-oxide corrosion deposits within nuclear steam generators and to prevent the formation of such corrosion deposits during the operation of nuclear steam generators.
It is still another object of the present invention to prepare a high-purity polymer dispersant using non-inorganic initiators, terminators, and neutralizers for use in preventing the formation of sludge, corrosion, or metal-oxide deposits within the secondary side of nuclear steam generators.
It is still another object of the present invention to prepare a high-purity polymer dispersant having a high purity and molecular weight sufficient to render the polymer dispersant thermally stable with sufficient dispersant activity under a pressure of 1300 psi or less and a temperature corresponding to the saturation temperature at 1300 psi.
It is still another object of the present invention to minimize system upsets in nuclear steam generators by neutralizing the high-purity polymer dispersant with amines, such as, monoethanolamine, morpholine, dimethylamine, 3-methoxypropylamine, diethanolamine, diethylaminoethanol, diemrthylpropanolamine, cyclohexylamine, 2-amino-2-methyl-1-propanol, triethanolamine, 3-hydroxyquinuclidine and 5-aminopentanol to maintain a pH level of about 9.5 in the steam generators.
It is still another object of the present invention to utilize a high-purity polymer dispersant mixed with the feedwater entering the secondary side of a pressurized water reactor steam generators operating in a pressure range of 500 to 1300 psi to prevent the formation of metal-oxide corrosion deposits during the operation of the nuclear power plant.
It is still another object of the present invention to utilize high-purity polymer dispersants which may contain sulfur-containing active groups or phosphorus-containing active groups which can be qualified to meet the necessary water quality specifications and which may be used to provide removal of metal oxides from the nuclear steam generator during shutdown.
It is still another object of the present invention to utilize methods such as filtration with specialized filter media by varying the effective pore sizes and zeta potential to remove residual polymer dispersant and complexed metal-oxide/polymer dispersant from the discharge blowdown water for recycling.
It is still another object of the present invention to utilize charcoal or activated carbon filters to remove residual polymer dispersant and complexed metal-oxide/polymer dispersant from the discharged blowdown water for recycling through the system or ultimate discharge to a receiving stream.
It is still another object of the present invention to utilize methods such as demineralization to remove the residual-polymer dispersant and complexed metal-oxide/polymer dispersant from the discharged blowdown water for use as recycled feedwater for steam generation within the nuclear plant or ultimate discharge to a receiving stream.
It is still another object of the present invention to utilize purification and ultrafiltration methods, such as flocculation, coagulation, reverse osmosis, and ultrafiltration to remove the residual polymer dispersant and complexed metal-oxide/polymer dispersant from the discharged blowdown water prior to recycling or ultimate discharge to a receiving stream.
The present invention relates to the utilization of selected high-purity polymer dispersants for preventing the formation of deposits of metal oxides within the secondary side of a nuclear steam generator in all modes of operation. The polymer dispersant is selected from a group consisting of acrylic acid polymer, methacrylic acid polymer, acrylate polymer, methacrylate polymer, copolymers, terpolymers, and mixtures thereof, acrylate/acrylamide copolymer, acrylate/methacrylate copolymer, terpolymers, and mixtures thereof. Specifically, the polymer dispersant may be the polymer monounsaturated carboxylic acid or the polymer sulfonated styrene polymer, and copolymers. Also, it is within the scope of the present invention that polymer dispersants or polymer-dispersant blends having sulfur-containing and phosphorus-containing functional groups or mixtures thereof may be utilized for iron transport and removal from the nuclear steam generator during shutdown or operation of the nuclear power reactor.
The polymer dispersant used in the present invention is of sufficient purity wherein the resultant chemical analysis of the discharge from the secondary side of the nuclear steam generator through the blowdown system yields a concentration of sodium, potassium, calcium, magnesium, chloride, sulfate, silicate, and phosphate ions of less than about 10 parts per billion of each ion in the blowdown during normal operation.
Additionally, the polymer dispersants utilized in the present invention as an additive to the feedwater entering the secondary side of a nuclear power generator possess metal-oxide dispersive and sludge conditioning characteristic of approximately 1 to 1,000 parts polymer dispersant to remove and prevent the buildup of one part iron oxide, the predominant metal oxide contained in the metal-oxide/sludge discharge from the nuclear steam generator. However, the polymer-dispersant concentration depends upon the amount of iron oxide in the feedwater stream and the concentration of the polymer initially used as the additive and is, preferably, within the range of 1 to 25 parts polymer versus one part iron oxide. The measured cation conductivity of the steam exiting the secondary side of the nuclear power generator and entering the turbine, corrected to 25xc2x0 C., should be equal to or less than 1.0 xcexcS/cm. Finally, the metal-oxide transport, or degree of the removal of iron oxide from the feedwater entering the secondary side of the nuclear power generator, as contained in the blowdown stream, is equal to or greater than a five percent increase than when the system does not contain the disclosed polymer dispersant or dispersants.
The invention further consists of certain novel features and chemical details hereinafter fully described, and illustrated in the accompanying drawing and particularly pointed out in the appended claims, it being understood that various changes and details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.